The present invention relates to printing with a drop-on-demand ink jet print head wherein ink drops are generated utilizing a drive pulse which is shaped to enhance the consistency of drop flight time from the ink jet print head to print media over a wide range of drop ejection rates.
Ink jet printers, and in particular drop-on-demand ink jet printers having print heads with acoustic drivers for ink drop formation are well known in the art. The principle behind an impulse ink jet of this type is the generation of a pressure wave in an ink chamber and subsequent emission of ink droplets from the ink chamber through a nozzle orifice as a result of the pressure wave. A wide variety of acoustic drivers are employed in ink jet print heads of this type. For example, the drivers may consist of a transducer formed by a piezoceramic material bonded to a thin diaphragm. In response to an applied voltage, the piezoelectric ceramic deforms and causes the diaphragm to displace ink in the ink chamber, which results in a pressure wave and the flow of ink through one or more nozzles. Piezoelectric drivers may be of any suitable shape such as circular, polygonal, cylindrical, annular-cylindrical, etc. In addition, piezoelectric drivers may be operated in various modes of deflection, such as in the bending mode, shear mode, and longitudinal mode. Other types of acoustic drivers for generating pressure waves in ink include heater-bubble source drivers (so called bubble or thermal ink jets) and electromagnet-solenoid drivers. In general, it is desirable in an ink jet print head to employ a geometry that permits multiple nozzles to be positioned in a densely packed array with each nozzle being driven by an associated acoustic driver.
U.S. Pat. No. 4,523,200 to Howkins describes one approach to operating an ink jet print head with the purpose of achieving high velocity ink drops free of satellites and orifice puddling and providing stabilized jet operation. In this approach, an electromechanical transducer is coupled to an ink chamber and is driven by a composite waveform including independent successive first and second electrical pulses of opposite polarity in some cases and separated by a time delay. The first electrical pulse is an eject pulse with a pulse width which is substantially greater than the second pulse width. The illustrated second pulse in the case where the pulses are of opposite polarity has an exponentially decaying trailing edge. The application of the first pulse causes a rapid volume reduction of the ink chamber of the ink jet head and initiates the ejection of an ink drop from the associate orifice. The application of the second pulse causes rapid volume expansion of the ink chamber and produces early break-off of an ink drop from the orifice. There is no suggestion in this reference of controlling the position of an ink meniscus before drop ejection and therefore problems in uniform printing at high drop repetition rates would be expected.
U.S. Pat. No. 4,563,689 to Murakami, et al. discloses an approach for operating an ink jet print head with the purpose of achieving different size drops on print media. In this approach, a preceding pulse is applied to an electromechanical transducer prior to a main pulse. The preceding pulse is described as a voltage pulse that is applied to a piezoelectric transducer in order to oscillate ink in the nozzle and the energy contained in the voltage pulse is below the threshold necessary to eject a drop. The preceding pulse controls the position of the ink meniscus in the nozzle and thereby the ink drop size. In FIGS. 4 and 8 of this patent, the preceding and main pulses are of the same polarity. In FIGS. 9 and 11, of this patent, these pulses are of opposite polarity. This patent also mentions that the typical delay time between the start of the preceding pulse to the start of the main pulse is on the order of 500 microseconds. Consequently, in this approach, drop ejection would be limited to relatively low repetition rates.
In addition, Murakami et al. is directed to controlling drop size and does not describe an ink jet that ejects drops with flight times substantially independent of the repetition rate. Moreover, there is no teaching or suggestion in Murakami et al. that a bipolar waveform with a wait period has a minimum energy content at the dominant acoustic resonant frequency of the ink jet.
Although these prior art devices are known, a need exists for an improved ink jet printer which is capable of effectively achieving uniform high quality printing, at high print rates.